The demand for wireless communication systems has grown steadily, and is still growing, and a number of technological advancement steps have been taken during this growth. In order to acquire increased system capacity and user data bit rate for wireless systems by employing de-correlated propagation paths, MIMO (Multiple Input Multiple Output) systems have been considered to constitute a preferred technology for improving the capacity and the user data bit rate. MIMO employs a number of separate independent signal paths, for example by means of several transmitting and receiving antennas.
Generally, a MIMO system utilizes de-correlated, or at least essentially de-correlated, transmitted signals. The meaning of the term “de-correlated signals” in this context is that the radiation patterns are essentially de-correlated. This is today made possible by means of spatial separation, i.e. having at least two antennas separated by 5-10 wavelengths, (calculated from the centre frequency of the frequency band for which the antennas are designed), normally in azimuth, in order to achieve low correlation between the signals at the antenna ports. These at least two antennas have at least one antenna radiation lobe each.
It is also possible to combine spatial separation with polarization separation, where the antennas then also are arranged for transmission and reception of signals having orthogonal polarizations.
A base station in a MIMO system may thus be arranged with a number of antennas, separated by 5-10 wavelengths, each one of the base station antennas either being designed for one polarization or a plurality of essentially de-correlated polarizations, typically two essentially de-correlated polarizations. These antennas produce antenna radiation lobes which are de-correlated, either by space or polarization, or both.
It is necessary that a user equipment (for example a mobile phone or a portable computer) is arranged with at least two antenna ports for communication in a MIMO system.
A problem with existing MIMO arrangements is that, since an original information stream is divided into two or more separate information streams, the SNR (Signal to Noise Ratio) is deteriorated given a fixed output power. A decreased SNR results in that the rate of transferred data, the data bit rate, is decreased.
Furthermore, the signal path between the base station and the user equipment may be blocked by a number of objects such as large buildings in an urban environment, which objects cause a number of reflections. These reflections may result in that the signal to noise ratio (SNR) becomes even more deteriorated, and thus the signals transmitted between the base station and the user equipment may become more and more noisy the more buildings that are in the way. It may thus be possible to use MIMO only in the vicinity of a base station. A good MIMO performance requires good SNR.
The traditional MIMO systems, having one information stream per antenna, are thus afflicted with a disadvantage concerning the data bit rate between the base station and the user equipment, both in urban environments and in the countryside, due to the fact that the base station antenna radiation lobes are spatially separated in order to obtain essentially de-correlated signals. This means that the MIMO system is not used optimally, for a given surface area and output power.